The present invention relates to a thin, forged magnesium alloy casing suitable as light, strong casings for small electronic appliances and media and a method for producing such a thin forged casing.
Because magnesium has the smallest specific gravity of 1.8 among metal materials put into practical use at present, magnesium alloys are finding wide expectations and applications as light, strong materials alternative to aluminum having a specific gravity of 2.7 and it alloys. Magnesium alloys may be used for parts of aircraft and spacecraft, land transportation equipment, cargo equipment, industrial machines and tools, electronic equipment, telecommunications equipment, agricultural machines, mining machines, office equipment, optical equipment, sports gear, etc.
The magnesium alloys are, however, much poorer in plastic working than aluminum alloys. Accordingly, the magnesium alloys are usually provided as die-castings at present. To improve castability and mechanical strength, magnesium is alloyed with aluminum, zinc, etc. Zirconium may be added to provide strength and toughness, and manganese may be added to make the crystal grains of the magnesium alloys finer. Also, rare earth elements and silver may be added to provide heat resistance.
However, magnesium alloy castings are limited to relatively thick products, because it is extremely difficult to cast magnesium alloys into thin products. In addition, casting defects such as pores and inclusions such as oxides, which are inevitable in casting, may be contained in the magnesium alloy castings and appear on the surface thereof. The casting defects and the inclusions deteriorate the mechanical strength of the magnesium alloy castings, and if they appear on the surface, they adversely affect the corrosion resistance and surface appearance of the castings.
Recently proposed and attracting attention is a so-called semi-solid method for forming magnesium alloy members in a temperature range in which a solid phase and a liquid phase coexist, by utilizing an injection technique. Products obtained by this forming method have fine crystal structures free from dendrites existing in usual castings, and also have higher density with fewer pores than die-castings, whereby they can be subjected to a heat treatment. This method can produce magnesium alloy members as thin as 1.5 mm or less. Nevertheless, the semi-solid, forming method is disadvantageous in that magnesium alloy members produced thereby are not necessarily free from defects and oxide inclusions inside and on the surface. With defects and oxide inclusions, good surface conditions such as appearance and corrosion resistance cannot be obtained.
Another method for forming thin magnesium alloy products is a drawing method. The drawing method comprises casting a magnesium alloy into an ingot; forging the ingot to remove or reduce defects and segregation; cutting or rolling the forged product to a proper length or thickness to form a thin plate; and drawing the thin plate to a desired shape. The drawing method is disclosed in Japanese Patent Laid-Open Nos. 6-55230 and 6-328155, Summary of the 89th Autumn Convention of the Light Alloys Association in 1995, pp. 179-180, etc.
Japanese Patent Laid-Open No. 6-55230 discloses that the deep drawing of a thin magnesium alloy plate can be carried out with a die with a punch and a flange portion heated to a surface temperature of 175-500xc2x0 C. In the Summary of the 89th Autumn Convention, a 1-mm-thick disc plate made of a magnesium alloy (AZ31) having a diameter of 60-65 mm is subjected to deep drawing with a punch having a radius of 40 mm and a shoulder radius of 12 mm and a die having a cavity having an inner diameter of 43 mm and a shoulder radius of 8 mm, at a blank pressure of 1000 kgf.
The deep drawing method, however, is only applicable to products having smooth surfaces, failing to provide products with projections. In addition, a smaller die shoulder radius than the above would cause cracking in the resultant products at inner bottom edges and corners, failing to provide products with sharp bottom edges and corners.
Because electronic circuits and elements are highly integrated and made denser recently, miniaturization and weight reduction are widely pursued in many applications such as mobile telecommunications gear such as cellular phones, note-type or mobile personal computers, electronic recording media such as compact disks, minidisks, etc. Casings for these appliances and media are mostly made of aluminum alloys at present, though further weight reduction is desired while keeping mechanical strength equivalent to or more than that of aluminum alloys. Magnesium alloys are promising because of their small specific gravity and high mechanical strength, if they can be forged into thin casings with sharp bottom edges, corners and projections.
Japanese Patent Laid-Open No. 6-172949 discloses a magnesium alloy part such as an automobile wheel, etc., and a forging method for producing such a magnesium alloy part. This forging method comprises (a) forging a magnesium alloy casting at a temperature of 300-420xc2x0 C. to form a forged part having an average crystal size of 100 xcexcm or less; and (b) subjecting the forged part to a T6 heat treatment comprising a solution treatment and an aging treatment. The forged part is subjected to finish working such as spinning and rolling. In a specific example, the above forging step (a) is carried out under the conditions that the magnesium alloy casting is heated at 400xc2x0 C., the die is heated at 250xc2x0 C., and the forging speed is 10 mm/sec. With an average crystal size of 100 xcexcm or less, the forged magnesium alloy has improved corrosion resistance and mechanical strength.
The technology proposed by Japanese Patent Laid-Open No. 6-172949 is, however, aimed at large, thick parts such as automobile wheels, etc., not coping with difficulty in forging extremely thin products with sharp bottom edges, corners and projections. It also requires the T6 heat treatment that takes a long period of time. If the technology of Japanese Patent Laid-Open No. 6-172949 is applied to forged casings of magnesium alloys, the resultant forged casings would not be able to be made as thin as 1.5 mm or less with sharp bottom edges, corners and projections, because the die at 250xc2x0 C. cools the magnesium alloy body too low to achieve smooth plastic flow (metal flow) of magnesium alloys during the forging.
Accordingly, an object of the present invention is to provide a light, thin, forged magnesium alloy casing with sharp bottom edges, corners and projections.
Another object of the present invention is to provide a light, thin, forged magnesium alloy casing with sharp bottom edges, corners and projections and substantially free from flow marks on the surface.
A further object of the present invention is to provide a method for producing such a light, thin, forged magnesium alloy casing precisely and inexpensively.
As a result of research in view of the above objects, the inventors have found the following facts leading to the completion of the present invention:
(1) Smooth metal flow can be achieved during the forging, if the magnesium alloy body to be forged is heated to a temperature near its melting point, while ensuring that the magnesium alloy is not melted locally by heat generated by strong friction.
(2) When the magnesium alloy is forged at a large compression ratio, remarkable flow marks appear on the surface of the resultant forged products.
(3) When a thin magnesium alloy plate is subjected to rough forging at a limited compression ratio, the surfaces of the magnesium alloy plate in contact with the die surface do not substantially flow, only the inside of the magnesium alloy plate plastically flows laterally. As a result, good surface conditions of the magnesium alloy plate are maintained.
Thus, the present invention provides a thin forged casing integrally constituted by a thin plate with projections on either or both surfaces, the plate being as thin as 1.5 mm or less. The thin plate constituting the thin forged casing is preferably as thin as 1 mm or less.
In a preferred embodiment, the thin forged casing is substantially free from flow marks on the surface.
In another preferred embodiment, the thin forged casing has sharp bottom edges and corners whose inner surfaces have radii of curvature of about 2 mm or less, particularly about 1 mm or less, and sharp projections whose shoulders have radii of curvature of about 2 mm or less, particularly 1 mm or less.
The present invention further provides a method for producing a thin, forged magnesium alloy casing comprising carrying out forging by at least two steps, a first forging step being to roughly forge a magnesium alloy body preheated at 350-500xc2x0 C. with a first die heated at 350-450xc2x0 C. to form an intermediate forged product; and a second forging step being to precisely forge the intermediate forged product preheated at 300-500xc2x0 C. with a second die heated at 300-400xc2x0 C.
In a preferred embodiment, the method for producing a thin, forged magnesium alloy casing integrally constituted by a thin plate of 1.5 mm or less in thickness with projections on either or both surfaces comprises (a) carrying out a first forging step for roughly forging a magnesium alloy plate preheated at 350-500xc2x0 C. with a first die heated at 350-450xc2x0 C. to form an intermediate forged product at a compression ratio of 75% or less; and (b) carrying out a second forging step for precisely forging the intermediate forged product preheated at 300-500xc2x0 C. with a second die heated at 300-400xc2x0 C. at a compression ratio of 30% or less.
The magnesium alloy plate to be roughly forged preferably has a thickness of about 3 mm or less. The first forging step is preferably carried out at a compression pressure of 3-30 tons/cm2 and a compression speed of 10-500 mm/sec. The second forging step is preferably carried out at a compression pressure of 1-20 tons/cm2 and a compression speed of 1-200 mm/sec.